Chiral Induction on the Ultrafast Event of Excited State Proton Transfer Can Probe Its Mechanism

Abstract

When protonic motion is explained as a purely quantum mechanical phenomenon, two extreme situations can be visualized as either the adiabatic or the nonadiabatic limit of proton transfer (PT). Strong orbital coupling between the donor and the acceptor pair in the proton transfer step drives PT mechanism in the adiabatic limit whereas in nonadiabatic limit PT predominantly occurs through tunneling. Here we proposed a unique way to experimentally observe the adiabaticity of PT by introducing chirality in the reaction coordinate of excited state proton transfer (ESPT). We successfully established the existence of prominent role of diastereomerism in the ESPT by both steady state and femtosecond time‐resolved transient absorption spectroscopic measurements in the adiabatic limit. The presented novel approach can overcome the limitations of only available experimental technique, i. e. kinetic isotope effect, to elucidate proton transfer mechanism.

Water‐Assisted Photoinduced Tautomerization in a Potential Bioactive Semicarbazone Schiff Base

“…The divergent emission spectra of DHHC in water may be due to its photoacidic character, where water molecules act as a base to extract acidic protons of phenolic hydrogen (O27−H36) and form hydronium ions (H 3 O + ). Consequently, the hydronium ion (H 3 O + ) generated during the photoexcitation of DHHC may undergo a solvent‐assisted ESPT process leading to the formation of the keto‐tautomer in the excited state (Scheme 2), which exhibits an emission maximum at 430 nm [24,26,48–51] . However, this phenomenon has not been observed in solvents such as alcohols, suggesting weak acidic nature of photoexcited DHHC [52] .…”

“…Consequently, the hydronium ion (H 3 O + ) generated during the photoexcitation of DHHC may undergo a solvent-assisted ESPT process leading to the formation of the keto-tautomer in the excited state (Scheme 2), which exhibits an emission maximum at 430 nm. [24,26,[48][49][50][51] However, this phenomenon has not been observed in solvents such as alcohols, suggesting weak acidic nature of photoexcited DHHC. [52] From the experimental data, it was observed that the change in the acidity constant ~pKa* is about 1.2, suggesting a weak photoacidic character (equation 1) of DHHC.…”